Conventionally, in the dental diagnosis, X-ray equipment, an intraoral camera, a dental camera, X-ray CT, MRI, etc. have been used for photographing a stomatognathic region.
An image obtained in the X-ray equipment is only a transmitted image, and information on an X-ray traveling direction of a subject is detected while being overlapped with each other. Therefore, the internal structure of the subject cannot be known three-dimensionally. Furthermore, an X-ray is harmful to a human body, so that an annual exposure dose is limited, and an X-ray is dealt with only by an eligible operator and is used only in a room surrounded by a shielding member of lead, lead glass, or the like.
The intraoral camera captures only the surface of an intraoral tissue, so that information on the inside of teeth and the like cannot be obtained. The X-ray CT is harmful to a human body in the same way as in the X-ray equipment. In addition, the X-ray CT has poor resolution, and the device is large and expensive. The MRI has poor resolution, and the device is large and expensive. In addition, the MRI cannot photograph the internal structure of teeth containing no moisture.
The optical coherence tomography device (hereinafter, referred to as an OCT device) is harmless to a human body, and enables three-dimensional information on a subject to be obtained with high resolution. Therefore, the optical coherence tomography device has been applied to the opthalmological field such as tomography of a cornea and a retina, etc. (see, for example, Patent Documents 1-4).
Hereinafter, a conventional OCT device will be described. FIG. 13 is a diagram showing a configuration of a conventional OCT device. In an OCT unit 1 constituting the OCT device shown in FIG. 13, light emitted from a light source 2 is collimated by a lens 3, and thereafter, is split into reference light 6 and measuring light 5 by a beam splitter 4. The measuring light 5 is condensed to a sample 10 to be measured by an objective lens 9 via a galvanomirror 8. Then, the condensed light is scattered and reflected from the sample 10, and passes through the objective lens 9, the galvanomirror 8, and the beam splitter 4 again to be condensed to a light photodetector 14 via a condensing lens 7. On the other hand, the reference light 6 passes through the objective lens 12 and is reflected from a reference mirror 13. Then, the reference light 6 passes through the objective lens 12 and the beam splitter 4 again, and thereafter is incident upon the condensing lens 7 in parallel with the measuring light 5 to be condensed to the photodetector 14.
The light source 2 is a low coherent in terms of time. Light beams emitted at different times from a light source that is low coherent in terms of time are very unlikely to interfere with each other. Therefore, an interference signal appears only when the distance of an optical path through which the measuring light 5 passes is substantially equal to that of an optical path through which the reference light 6 passes. Consequently, when the intensity of an interference signal is measured by the photodetector 14 while a difference in optical path length between the measuring light 5 and the reference light 6 is changed by moving the reference mirror 13 in an optical axis direction of the reference light 6, a reflectance distribution in a depth direction (z-axis direction) of the sample 10 can be obtained. That is, the structure in the depth direction of the sample 10 is obtained by optical path length difference scanning.
The two-dimensional cross-sectional image of the sample 10 is obtained by performing scanning in a horizontal direction (x-axis direction) by the galvanomirror 8 in addition to scanning in the depth direction (z-axis direction) of the sample to be measured by the reference mirror 13. In such an OCT device, measurement with resolution as high as several μm can be performed. Thus, the OCT device enables an image with high resolution of the inside of a living body to be obtained in a nondestructive and noncontact manner.
Regarding the application of the OCT device to the dental field, an example is disclosed in which a dental tomogram is taken using an OCT device (see, for example, Non-Patent Documents 1-5).    Patent Document 1: JP 2003-329577 A    Patent Document 2: JP 2002-310897 A    Patent Document 3: JP 11-325849 A    Patent Document 4: JP 2001-059714 A    Non-Patent Document 1: Laser review October, 2003: Technical development of optical coherence tomography for clinical application    Non-Patent Document 2: Journal of Biomedical Optics, October 2002, Vol. 7 No. 4: Imaging caries lesions and lesion progression with polarization sensitive optical coherence tomography    Non-Patent Document 3: APPLIED OPTICS, Vol. 37, No. 16, 1 Jun. 1998: Imaging of hard- and soft-tissue structure in the oral cavity by optical coherence tomography    Non-Patent Document 4: OPTICS EXPRESS, Vol. 3, No. 6, 14 Sep. 1998: Dental OCT    Non-Patent Document 5: OPTICS EXPRESS, Vol. 3, No. 6, 14 Sep. 1998: In vivo OCT Imaging of hard and soft issue of the oral cavity